I've just shot a semi-scientific (heavy tripod, cable release, USAF test chart at 2 metres) comparison of the Summar, Summicron DR and Elmar red scale at f/2 (where applicable), f/4 and f/8. I'll post the results when the film is developed but I *think* the Summar may surprise everyone at f/4 and f/8, although the Summicron DR is legendary.
One pleasing thing is that all three lenses focused exactly the same with the rangefinder. Originally I was going to scale focus, using a tape measure from the film plane, but since there was perfect agreement (and I've previously checked the rangefinder with at least one of the lenses and a tape measure), I decided the focus was accurate enough anyway. The M focusing patch (sorry, I know this is an LTM board) is much more accurate if you use it as a split-image rather than as a coincidence device; the latter is very inexact.
My unscientific opinion based on shooting with the Summar is that it is the sharpest of my 50mm lenses - right down to film grain level at f/4. Here's a small unsharpened 100% scan at 3600ppi. That's a six FOOT by four FOOT print.